The present invention relates to metal oxide semiconductor (MOS) imaging arrays and, more particularly, to a method for correcting for dead pixels in MOS imaging arrays.
Integrated circuit technology has revolutionized various fields including computers, control systems, telecommunications, and imaging. In the field of imaging, the charge coupled device (CCD) sensor has made possible the manufacture of relatively low-cost and small hand-held video cameras. However, the solid-state CCD integrated circuits needed for imaging are relatively difficult to manufacture, and therefore are expensive. In addition, because of the differing processes involved in the manufacture of CCD integrated circuits relative to MOS integrated circuits, the signal processing portion of the imaging sensor has typically been located on a separate integrated chip. Thus, a CCD imaging device includes at least two integrated circuits: one for the CCD sensor and one for the signal processing logic.
An alternative low-cost technology to CCD integrated circuits is the metal oxide semiconductor (MOS) integrated circuit. Not only are imaging devices using MOS technology less expensive to manufacture relative to CCD imaging devices, for certain applications MOS devices are superior in performance. For example, the pixel elements in a MOS device can be made smaller and therefore provide a higher resolution than CCD image sensors. In addition, the signal processing logic necessary can be integrated alongside the imaging circuitry, thus allowing for a single integrated chip to form a complete stand-alone imaging device.
Examples of MOS imaging devices are detailed in xe2x80x9cA xc2xc Inch Format 250 K Pixel Amplified MOS Image Sensor Using CMOS Processxe2x80x9d by Kawashima et al., IEDM 93-575 (1993), and xe2x80x9cA Low Noise Line-Amplified MOS Imaging Devicesxe2x80x9d by Ozaki et al., IEEE Transactions on Electron Devices, Vol. 38, No. 5, May 1991. In addition, U.S. Pat. No. 5,345,266 to Denyer titled xe2x80x9cMatrix Array Image Sensor Chipxe2x80x9d describes a MOS image sensor. The devices disclosed in these publications provide a general design approach to MOS imaging devices.
The primary building block of an image formed by an MOS imaging device is a pixel. The number, size, and spacing of the pixels determine the resolution of the image generated by the imaging device. The pixels of a MOS imaging device are semiconductor devices that transform incident light photons into current signals at an array cell, which may be a diode or gate cell. The signal produced by each pixel is generally extremely small, in the nanoampere range.
Although generally quite reliable, the pixels may from time to time exhibit anomalous behavior. For example, if the array cell of the pixel has a defect in its PN junction, it may generate current leakage paths, thus causing the cell site to appear as a white spot in the image signal. The pixel of such a cell site may be referred to as a xe2x80x9cdead pixel.xe2x80x9d The number of dead pixels on a CMOS image sensor is dependent on the quality of the process used for forming the image sensor. In a high resolution sensor that has one million or more pixels, a number of dead pixels invariably result.
The present invention is directed to a method and apparatus that compensates for the foregoing and other problems of the prior art. More specifically, the present invention is directed to a method and apparatus for correcting for dead pixels in a CMOS image sensor.
A method for correcting for dead pixels in a CMOS image sensor is disclosed. According to the method, when the image sensor is first powered on, the pixel array is examined for dead pixels and the locations of any dead pixels that are found are digitally stored. Thereafter, as a given frame of the image sensor is scanned out, the location of the pixels in the frame are compared to the stored locations for dead pixels, and if a dead pixel is determined, the dead pixel signal is compensated for. One method of compensating for a dead pixel signal is to replace it with the pixel signal from a previous pixel, or an average of the previous and following pixels.